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<head>
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  <meta name="viewport" content="width=device-width, initial-scale=1.0, user-scalable=yes">
  <title>WebGL - Fundamentals</title>
  <link type="text/css" href="../resources/webgl-tutorials.css" rel="stylesheet" />
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<body>
  <div id="info">
    <a href="https://webglfundamentals.org/webgl/lessons/zh_cn/webgl-3d-perspective-correct-texturemapping.html"
      target="_blank">WebGL 纹理映射的透视纠正</a>
  </div>
  <canvas id="c"></canvas>
</body>

</html>
<!--
for most samples webgl-utils only provides shader compiling/linking and
canvas resizing because why clutter the examples with code that's the same in every sample.
See http://webglfundamentals.org/webgl/lessons/webgl-boilerplate.html
and http://webglfundamentals.org/webgl/lessons/webgl-resizing-the-canvas.html
for webgl-utils, m3, m4, and webgl-lessons-ui.
-->
<script src="../resources/webgl-utils.js"></script>
<script id="2d-vertex-shader" type="notjs">

  // 属性从缓冲中获取数据
  attribute vec4 a_position;

  // 所有的着色器都有一个 main 函数
  void main() {

    // gl_Position 是着色器需要设置的一个特殊的变量
    gl_Position = a_position;
  }

</script>

<script id="2d-fragment-shader" type="notjs">

  // 片断着色器没有默认的精度，
  // 中等精度是个不错的默认值
  precision mediump float;

  void main() {
    // gl_FragColor 是片断着色器需要设置的一个特殊变量
    gl_FragColor = vec4(1, 0, 0.5, 1); // 返回红紫色
  }

</script>
<script>
  "use strict";

  function main() {
    // Get A WebGL context
    var canvas = document.getElementById("c");
    var gl = canvas.getContext("webgl");
    if (!gl) {
      return;
    }

    // setup GLSL program
    var program = webglUtils.createProgramFromScripts(gl, ["2d-vertex-shader", "2d-fragment-shader"]);

    // look up where the vertex data needs to go.
    var positionAttributeLocation = gl.getAttribLocation(program, "a_position");

    // Create a buffer and put 12 clip space points in it.
    // 4 rectangles, 2 triangles each, 3 vertices per triangle
    var positionBuffer = gl.createBuffer();

    // Bind it to ARRAY_BUFFER (think of it as ARRAY_BUFFER = positionBuffer)
    gl.bindBuffer(gl.ARRAY_BUFFER, positionBuffer);

    var positions = [
      -.8, .8, 0, 1,  // 第一个矩形的第一个三角形
      .8, .8, 0, 1,
      -.8, .2, 0, 1,
      -.8, .2, 0, 1,  // 第一个矩形的第二个三角形
      .8, .8, 0, 1,
      .8, .2, 0, 1,

      -.8, -.2, 0, 1,  // 第二个矩形的第一个三角形
      .8, -.2, 0, 1,
      -.8, -.8, 0, 1,
      -.8, -.8, 0, 1,  // 第二个矩形的第二个三角形
      .8, -.2, 0, 1,
      .8, -.8, 0, 1,
    ];
    gl.bufferData(gl.ARRAY_BUFFER, new Float32Array(positions), gl.STATIC_DRAW);

    // code above this line is initialization code.
    // code below this line is rendering code.

    webglUtils.resizeCanvasToDisplaySize(gl.canvas);

    // Tell WebGL how to convert from clip space to pixels
    gl.viewport(0, 0, gl.canvas.width, gl.canvas.height);

    // Clear the canvas
    gl.clearColor(0, 0, 0, 0);
    gl.clear(gl.COLOR_BUFFER_BIT);

    // Tell it to use our program (pair of shaders)
    gl.useProgram(program);

    // Turn on the attribute
    gl.enableVertexAttribArray(positionAttributeLocation);

    // Bind the position buffer.
    gl.bindBuffer(gl.ARRAY_BUFFER, positionBuffer);

    // Tell the attribute how to get data out of positionBuffer (ARRAY_BUFFER)
    var size = 4;          // 4 components per iteration
    var type = gl.FLOAT;   // the data is 32bit floats
    var normalize = false; // don't normalize the data
    var stride = 0;        // 0 = move forward size * sizeof(type) each iteration to get the next position
    var offset = 0;        // start at the beginning of the buffer
    gl.vertexAttribPointer(
      positionAttributeLocation, size, type, normalize, stride, offset);

    // draw
    var primitiveType = gl.TRIANGLES;
    var offset = 0;
    var count = 4 * 3;   // 4 triangles, 3 vertices each
    gl.drawArrays(primitiveType, offset, count);
  }

  main();
</script>